The Proto-Slavic accent is the accentual system of Proto-Slavic and is closely related to the accentual system of some Baltic languages (Lithuanian and Latvian) with which it shares many common innovations that occurred in the Proto-Balto-Slavic period. Deeper, it inherits from the Proto-Indo-European accent. In modern languages the prototypical accent is reflected in various ways, some preserving the Proto-Slavic situation to a greater degree than others.

Proto-Balto-Slavic is reconstructed with a free lexical accent, and a distinction between "short" and "long" syllables. A long syllable was any syllable containing either a long vowel, a diphthong, or a so-called "sonorant diphthong" consisting of a short vowel plus *l, *m, *n or *r in the syllable coda. Short syllables consisted of a short vowel with either no coda or an obstruent in the coda. The distinction between long and short syllables remained important throughout the early history of Slavic.

Long vowels were present in Proto-Balto-Slavic, and remained in Proto-Slavic as well. However, alongside the distinctions in quantity, Slavic also developed distinctions in quality between short and long vowels:

This became important towards the end of the Proto-Slavic period, where certain dialects acquired new long and short vowels distinct from the older ones, based on these differences in quality.

Proto-Balto-Slavic long syllables could bear a suprasegmental feature known as acute. The acute feature could occur independently of the accent, and Slavic retained this situation until at least the operation of Dybo's law. This sound change shifted the accent one syllable rightwards if it previously fell on a non-acuted syllable (whether short or long), and the syllable that the accent shifted onto still had the acute–nonacute distinction at this time. However, by the time of Ivšić's law, the acute feature was no longer apparent; the accent retraction that occurred as part of this law produced the same result on all long syllables, regardless of whether it was originally acuted or not. The Slavic accent had changed from being purely positional to also being intonational: the acute was converted to a distinct rising intonation on accented long syllables, and lost elsewhere. Unaccented syllables now distinguished only between long and short.

Slavic also inherited from Proto-Balto-Slavic the distinction between fixed and mobile accentual paradigms in verbs and nominals. In fixed paradigms, the accent was on the same stem syllable in all forms, while in mobile paradigms, it would alternate between the first syllable of the stem and the ending. Fixed paradigms were split in two by the operation of Dybo's law, which created a new accent paradigm b by shifting the accent onto the ending. The accent was then shifted back again in some forms by Ivšić's law, creating a new type of mobile pattern. Paradigms which remained fixed were assigned to accent paradigm a. The inherited Balto-Slavic mobile paradigms were not split in this way thanks to Meillet's law, and remained unified in accent paradigm c.

There is no consensus among linguists on the exact prosodical nature of late Proto-Slavic, or Common Slavic. Two different schools of thought exist, the more-or-less "traditional" school (exemplified by Jasanoff, Kapović and Olander among others) and the more radical "Leiden" school (exemplified by Derksen, Kortlandt and Pronk among others). The most important difference between the two, in the context of the Slavic accent and prosody in general, is that the traditional viewpoint holds that Proto-Slavic retained all length distinctions as they were inherited from Proto-Balto-Slavic, whereas the Leiden school argues that some long vowels were shortened and short vowels were lengthened already in Proto-Slavic. Thus:

The various accent types of Proto-Slavic are indicated with different diacritical symbols. The following table helps map between the notational systems found in various sources. The vowel symbols stand for different vowel classes that the accent can appear on: o stands for any original short mid vowel, ъ stands for any original short high vowel (yer), a for any original long vowel or liquid diphthong.